Remote alpine lakes affected by nitrogen from agriculture transported across vast distances in the atmosphere

Remote mountain lakes in Utah, USA receive significant amounts of nutrients transported in the atmosphere from human activities many miles away, according to a new study. High alpine lakes are generally nutrient poor, and so this atmospheric arrival of nutrients – largely originating from nitrate and ammonium based fertilisers used in agriculture – has the potential to significantly alter the health and functioning of their ecosystems.

In order to protect these remote ecosystems, which often provide water resources for nearby lowlands and hotspots for rare and endemic biodiversity, it is important to identify the sources of nitrogen that reach them.

Reactive nitrogen (given the chemical symbol Nr) is the name given to all forms of biologically available nitrogen, including inorganic forms such as ammonia, ammonium, nitrogen oxide and nitrates; and organic compounds such as urea and nucleic acids. Reactive nitrogen is added to the environment naturally by lightning strikes and nitrogen fixation.

However, human activities such as synthetic fertiliser production and fossil fuel contributions have doubled levels of reactive nitrogen in the Earth’s nitrogen cycle. The emissions from such nitrogen-producing activities may be transported and deposited hundreds, even thousands, of miles from their source.

Mirror Lake in the Uinta Mountains. Image: Bryant Olsen | Flickr

The team analysed three stable isotopes (Δ17O, δ18O and δ15N) sampled in water from lakes in the Uinta Mountains in northeastern Utah, USA. This analysis allowed the team to determine where the nitrates found in the lakes – which are remote, with little direct human impact – originate from and how they were transported. For example, the isotope samples allowed the team to differentiate between nitrates originating from fossil fuel burning, biomass burning and lightning that is oxidised in the atmosphere; and those which are oxidised in land and water ecosystems.

Their results show that at least 70% of the total nitrate inputs into the Uinta Mountain lakes originate from the atmosphere. The majority of the nitrates arriving into the lakes are the result of agricultural activities, specifically the use of nitrate and ammonium fertilisers. The research team suggest that, “similarities in nitrate isotope compositions between Uinta Mountain lakes and lakes throughout the US Rocky Mountains suggest that these findings apply to other mountain regions in western North America.”

These findings are significant because they highlight how nitrate-caused ecological stress in mountain lakes may be the result of from multiple sources of nitrogen located many miles from the affected ecosystems. In short, human activities such as intensive agriculture have the potential to negatively impact even the most seemingly remote or ‘wild’ places.

For environmental managers and policy makers seeking to conserve these ecosystems, the study suggests that the geographical range of potential nitrate sources must be widened if nitrate levels are to be managed and their effects mitigated. Here, local ecological stresses are inextricably tied to wider-scale human activities. As such managing the emission and transportation of nitrates across vast distance is likely to pose ever more complex challenges for environmental policy and conservation.

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